After additional testing, it turns out that it also happens when the player is part of the group, and the AI leader will actually order the group to move 200 meters away after a "Get out" waypoint.
The problem is also not consistent. Sometimes they will just get out, other times they'll run off hundreds of meters.
And it's definetly due to the "Get out" waypoint, 'cause if you put a "Move" waypoint right at the "Get out" waypoint, they'll wander off, then come back to the move waypoint.
Pure AI group: http://youtu.be/M1vuMUffGAk
Player as sub-unit: http://youtu.be/idFaYyFJEgQ